M4A1 vs M4A1 CQB

I was just wondering, what are the internal differences between regular and CQB versions of M4A1? I know the CQB version was tuned down, but I'm mainly interested in the steps taken in the process of tuning down.

OK now it makes sense. When it comes to the higher ROF, the CQB shoots faster mainly due to the fact, the opening on the side of the cylinder is located lower than on the regular M4 cylinder, therefore the piston is responsible for a smaller volume of air being pushed through. In this situation there is less strain being put on the spring and the whole system can work faster. For example, installing a Type0 cylinder( without any openings) would require the spring and the piston to push much more air through the nozzle and result in slower ROF.